The shift rods aren't identical and they are specific to the Getrag in the W123 240D. You'll also need the Getrag shifter. The bulk of the shifter assembly is the same but the shift arms are different.

The real show stopper is the intermediate plate mentioned by Mr. Reiner. You will need to replace that plate. The Getrag requires dowel holes for alignment. The later intermediate plate will accommodate the Getrag and the iron box but your early plate will only work with the iron box. I have a 1977 240D intermediate plate on the shelf and it will not work with the Getrag. I found out all about this when I put a Getrag 5 speed behind a 74 OM616.

To change that plate you have to remove the flywheel and you may need to replace the 12 expensive torque to yield bolts holding it on. These bolts can be reused if they meet specs given in the service manual. The flywheel must be marked relative to the crank when removed. There is also a factory centering procedure for the replacement plate.

Getrags use a different clutch slave and clutch line routing.

If I was in your shoes I'd source another iron 4 speed. You can find the slope back iron 4 in 74-80 240Ds. The notch back iron 4 will also fit and was used in 68-73 W115/114 cars. You'll spend much more time and money trying to fit the Getrag to your early car. Exploding iron 4 speeds are far from the norm. Many consider them better than the Getrag.

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1969 220D 5 Speed (OM616)
1983 240D 4 Speed
1985 300D Auto 376K
1985 300D Auto 275K